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[Dracoro]

Had top end work done a while ago (rocker gaskets, HBV, sparks etc.). I mentioned coolant loss and could they investigate. They said it was thermostat leaking and replaced that.

Anyway, it's still losing coolant. about 1 litre every few hundred miles.

No mayo, no noticeable overheating, no obvious leaks on floor, can't see any leaks in engine bay (which was cleaned up when top end work done so should be able to see everthing) etc.

Before I take it back and ask why the garage didn't fix the leak, is there any common leak points on the 2.5 V6 petrol that I can check?

Could it be leaking onto a hot manifold or suchlike hence not seeing the leak?

Car has only done genuine 66k, had numerous issues, not impressed by VX engineering, shoulda gone jap Anyway.....

BTW, other than this a lot of stuff done so if anyone wants to buy a 66k Omega Elite facelift.........

[Entwood]

Having checked all the obvious things .. check the header tank/cap. It is not that uncommon that the header tank develops tiny cracks around the filler, or that the cap does not seal correctly, or that the safety valve in the cap leaks pressure.

Any of these and the system does not pressurise properly, allowing the system to vent water vapour, basically the hot water in the header tank "evaporates" quite rapidly at just below boiling point.

Worth a thought - new cap is pretty cheap 

[Dracoro]

Will check the cap. Header tank is new(ish) though.

Would a faulty cap really lose that much vapour though?

[ScaniaV8]

Althogh you state the HBV was replaced I would double check HBV operation is definitely OK as if replaced may not have had hoses/clips attached securely. Amount of leakage seems like HBV to me.
However the best way to check is of course doing a 'pressure test 'with the Plenum and inlet manifold removed to view whole 'V' section. There is a good guide in the Maintenance section. 

[Dracoro]

Thanks chaps.

I don't have a spare cap to butcher unfortunately.

Is it best to check for leaks when cold or warm/hot. i.e when is there more pressure, when warm or cold?.

If it is the HBV, how can I see/check for leaks since it's behind the block?

[Dracoro]

A quick update.

Uses 1ml per mile. I've just topped it up with about half a litre, last topped up 500 miles ago.

No oil in water, no water in oil. No overheating, nothing untoward at all really (apart from the coolant loss). No leaks that I can see (at the rate it's losing coolant, I doubt it would be visible anyway).

HBV not really that accessible to see if that's the problem, let alone tighten up any clamps etc.

I may get a replacement cap just in case - anyone know how much these are?

I guess, until there's an issue with oil/water, topping it up every 500 miles is no great hardship as no real damage being done etc....

[Entwood]

I was losing a fair bit more than that .. it was eventually solved by simply a new header cap ..   For the cost it has to be worth a try 

You have looked at the header tank neck Not unknown for tiny cracks to appear there .. but I think you'd lose a tad more from a crack than a faulty pressure relief ..